Microelectronic devices, such as ultra large-scale integrated (ULSI) circuits, are commonly formed as multi-layered devices having alternating layers of conductors and dielectric material. Each of these layers is deposited separately and often the layers are polished to a high degree of planarity prior to the deposition of an overlying layer. Chemical mechanical polishing is the leading process used to produce planar multi-layer metallization systems in modem ULSI circuits.
Prior to depositing a metal interconnect or conductor layer, a relatively thick dielectric layer is deposited over a substrate and any integrated circuit devices formed on the substrate. The dielectric layer is then polished using a chemically active slurry and a polishing pad to produce a very flat or planar surface. Contact holes or vias are etched in the dielectric material. A barrier metal and a tungsten film are then deposited over the etched dielectric in order to fill the vias. The tungsten film is then polished off the surface leaving a flat surface with the contact holes or vias filled with plugs of the barrier metal and tungsten. The metal interconnect layer is then deposited over the polished dielectric layer, forming electrical connection with the tungsten plugs.
A known difficulty with CMP operations is that the rate of material removal may be uneven across the surface of the wafer. U.S. Pat. No. 5,873,769 issued to Chiou, et al., describes a method and apparatus for achieving a uniform removal rate across the surface of a wafer. The Chiou patent addresses only variations in removal rates resulting from varying mechanical conditions across the wafer, and it does not address variations in removal rates resulting from changes in material characteristics of the wafer material.
The duration of a CMP process must be carefully controlled to ensure that a sufficient amount of material is removed without removing an excess amount of material. Various endpoint detection techniques are used to determine the proper time to terminate a CMP process. One endpoint detection technique involves following a power curve as the resistance on the platen increases as the polishing process progresses. This process relies on the simplifying assumption that the material removal rate is constant across the depth of the material being removed. In actuality, the instantaneous removal rate of a particular material may vary across its depth, and the resistance curve may become either dilated or eroded with respect to time. This may cause the proper endpoint to be missed.